Embodiments presented herein relate generally to a field of electrical machines and, more particularly, to an interior permanent magnet (IPM) rotor of the electrical machines.
An IPM machine (a type of “electrical machine”) is known for its high drive efficiency. Because of the efficiency, the IPM machine has been extensively used as a most common motor type in hybrid electric automobiles. The usage of this electrical machine has increased significantly over years.
The electrical machine comprises a stator and an IPM rotor. The rotor includes embedded permanent magnets. These magnets are restrained to rotor body against centrifugal forces using bridges or webs of the rotor body material. These webs and/or bridges provide structural strength to the rotor. Typically, these webs and bridges are made of magnetically conductive materials.
Efficiency and power delivery of this electrical machine is based on amount of flux transferred between the rotor and the stator. Leakage of flux around the magnets within the rotor limits the efficiency and power density of the motor by decreasing the flux transfer between the rotor and stator. The webs and/or bridges that provide structural support provide leakage paths for flux.
Known techniques discuss flux leakage reduction by means of reducing cross section area of webs and/or bridges. The flux leakage in these techniques is limited by maximum flux density that can be carried by the webs and/or bridges known as saturation flux density. Other techniques discuss flux leakage reduction by introduction of geometric irregularities, such as holes or slots in the webs and/or bridges. In this case, the total flux leakage would still be governed by the ability to limit the size of the magnetically conductive webs or bridges. In both the cases, the strength of webs and/or bridges that provide structural support and strength is reduced. Moreover, reduction in structural support increases mechanical stress in the webs and/or bridges during operation of the rotor. In order to maintain mechanical integrity in operation, webs and/or bridges of sufficient size and strength need to be provided to limit the mechanical stress. However, providing webs and/or bridges of sufficient strength and size causes an increase in the leakage of flux. This flux leakage significantly reduces the machine power density, and the machine efficiency.
Therefore, there is a need for reducing the flux leakage in the rotor without compromising structural integrity of the rotor assembly.